1. Field of the Invention
The present invention relates to a channel allocating method for random access in uplink of a radio access network.
2. Discussion of the Related Art
Generally, a radio access network according to a related art consists of an access point (hereinafter abbreviated AP) and a mobile terminal (hereinafter abbreviated MT) attempting to access the AP via an air interface. In this case, FDMA, TDMA, CDMA, OFDMA, OFDMA-CDMA or the like can be used as the air interface.
In case that the MT attempts to transmit data in uplink (from MT to AP), an uplink resource needs to be allocated to the MT. To have the uplink resource allocated, the MT transmits a resource request packet (hereinafter abbreviated RR) to the AP via a random access channel (hereinafter abbreviated RACH).
Each MT that requests the uplink resource transmits the RR packet via RACH each frame until receiving a response of access success or failure from the AP. In this case, the frame is a medium access control 9 hereinafter abbreviated MAC) frame of a limited count.
Meanwhile, a result of an access attempt, i.e., the access success or failure (collision) is notified to the corresponding MT via an access feedback channel (hereinafter abbreviated AFCH) among downlink (from AP to MT) channels of a next frame.
Collision frequently occurs in the above-explained random access process since at least one or more MTs make access via the same RACH. Each of the MTs having the collision occurrences in the random access process should retransmit the RR packet to the AP. In this case, there are three RACH allocating methods according to the AP's scheme of processing the retransmitted RR packet as follows.
1. First RACH Allocating Method that AP Fixes the Count of RACH to Random ‘n’, [1]: HIPERLAN Type2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions, Broadband Radio Access Networks (BRAN), ETSI TS 101 761-1 V1.3.1, December (2001).
If a collision occurs when an MT transmits an RR packet to an AP via RACH, a window size (window size=frame count) is determined by binary exponential back-off algorithm according to the count of collision occurrences. The MT stands by during the determined window size and then retries a random access. Namely, after the duration of the frames of which count have been determined by the algorithm, the MT retransmits the RR packet to the AP via RACH.
Meanwhile, the random access via RACH is controlled by a contention window (hereinafter abbreviated CW) managed by each MT.
Each MT determines a size of CW by a retransmission count (a) of the RR packet according to the following manner.
First of all, the retransmission count in initial attempt is ‘0’. Hence, CW0 has a size corresponding to a RACH count (n) of a current frame.
On the other hand, the retransmission count (a) in retransmission has a value equal to or greater than ‘1’. Hence, the size of CW becomes 256 (2a≧256), 2a (n<2a≦256) or n (n≧2a) according to the retransmission count (a) of the RR packet for access re-attempt.
Meanwhile, for the control of random access via RACH, the MT determines the size of CW using the retransmission count (a) of the RR packet and then selects a uniform distribution random variable (r) between the CW sizes according to ‘1’ and the retransmission count (a). In this case, the selected ‘r’ becomes the number of RACH the MT attempts to access.
In the first allocating method, the count (n) of RACH in each frame is not changed.
2. Second Method of Allocating RACH Dynamically Using Split Algorithm:
[2] Peter Mathys and Philippe Flajolet, “Q-ary Collision Resolution Algorithm in Radio-Access Systems with Free or Blocked Channel Access”, IEEE Transactions on Information Theory, Vol. IT-31, No.2, March 1985;
[3] D. Petra, A. Kramling, and A. Hettich, “MAC Protocol for Wireless ATM: Contention Free versus Contention Based Transmission of Reservation Requests”, 7th IEEE PIMRC, 1996; and
[4] Benny Van Hooudt and Chris Blondia, “Analysis if an Identifier Splitting Algorithm Combined with Polling (ISAP) for Contention Resolution in a Wireless Access network”, IEEE Journal on Selected Area in Communications, Vol.18, No.11, November 2000.
FIG. 1 is a diagram of an example of allocation of random access channel (RACH) according to a related art, in which RACHs are allocated dynamically using split algorithm.
Referring to FIG. 1, after MTs of a set Qi have attempted random accesses via RACHs of a frame t, a contention resolution period corresponding to prescribed frames following the frame t is given to MTs of collision occurrence among the MTs of the set Qi. In this case, ‘Qi’ represents a set of MTs having made random access to a frame i.
In the second allocating method, the count of RACH differs in each frame.
And, the second allocating method is explained in detail as follows.
First of all, MTs of a set Qt having attempted random access via transmission of RR packet in the frame t are divided into MTs succeeding in access and MTs failing (colliding) in access.
The MTs (succeeding in access) having collision occurrence of the transmitted RR packets among the MTs of Qt acquire uplink resources from AP, i.e., prescribed channels.
In contrast, for the MTs (failing in access0 having the collision occurrence of the transmitted RR packets among the MTs of Qt, a contention resolution period begins in the AP.
Meanwhile, the RACHs of the frame t, in which the access-failing MTs have transmitted the RR packets, i.e., the RACHs having the collision occurrence are split into m (≧2) in a next frame (t+1). For instance, if collisions occur in RACHs of c, the count of RACHs in the frame (t=1) becomes (c×m).
Hence, the MTs of ‘j (≧2)’ having the collision occurrence in attempting the random access to a random kth RACH in the frame t re-attempt random access via one of ‘m’ RACHs from which the kth RACH was split in a next frame (t+1). Namely, the kth RACH of the frame t is split into ‘m’ in the next frame (k+1) and the RR packet is transmitted via one of the m-split RACHs in the next frame (t+1).
Yet, if collision occurs in c′ RACHs in the frame (t+1), the RACH count in a frame (t+2) becomes c′×m.
Therefore, ‘j’ MTs having the collision occurrence in attempting the random access in the frame t can send RR packets after a frame p(≧1).
In the above-explained second allocating method, throughput (=count of RACHs succeeding in access/total count of RACHs allocated to one frame) becomes higher than that of the first allocating method. And, the retransmission time of the second allocating method for the MTs having the collision occurrence is shorter than that of the first allocating method.
3. Third Method of Allocating RACH Dynamically by Algorithm of Equation 1, [5]
[5] Gyung-Ho Hwang and Dong-Ho Cho, “Adaptive Random Channel Allocation Scheme in HIPERLAN Type2”, IEEE Communications Letters, Vol.6, No.1 January 2002.r(t+1)=r(t)+α(Nf(t)−Ns(t))(1−Iidle(t))−Iidle(t)   [Equation 1]                r(t): Count of RACHs allocated to frame t        α weight        Nf(t): Count of RACHs colliding in random access in a frame t        Ns(t): Count of RACHs succeeding in access in a frame t        Iidle(t): ‘1’ if no access to RACH in a frame t, ‘0’ if not        
The RACHs of the frame t are classified according to the result of the random access attempt of MTs, i.e., according to the success or failure (collision) of the random access.
In the third method, new RACHs amounting to the count of RACHs having the collision occurrence among total RACHs of the frame t are added to the frame (t+1). On the contrary, RACHs amounting to the count of RACHs succeeding in access among total RACHs are deleted from the frame (t+1).
For instance, if the count r(t) of RACHs of frame t is 10, if collisions occur in three of the RACHs, if five of the RACHs succeed in access, and if the rest two of the RACHs did not attempt random access, a count r(+1) of RACHs that will be dynamically allocated to the frame (t+1) is ‘10+3−5=8’.
Meanwhile, a minimum count of RACHS should be at least 1 each frame. Hence, if the count r(t+1) of RACHs of the frame (t+1) is smaller than 1, it becomes ‘r(t+1)=1’.
If there was no access attempt to RACHs of the frame t, he count r(t+1) of RACHs of the frame (t+1) becomes ‘r(t)−1’.
And, the MTs having the collision occurrence in RR packet transmission determine the size of CW by the retransmission count (a) of the RR packets like the first allocating method and then retransmit the RR packets.
In the third allocating method, the count of RACHs is variable each frame. And, throughput (=count of RACHs succeeding in access/total count of RACHs allocated to one frame) becomes similar to or slightly higher than that of the first allocating method. Yet, the retransmission time of the third allocating method for the MTs having the collision occurrence is longer than the retransmission time of the first allocating method.
However, the related art allocating methods have the following problems or disadvantages.
In the first allocating method, since the count of the RACHs is the same in each frame, in case that the MTs attempting the random access are in a fluidly situation, waste or shortage of channel resources may be brought about. Hence the throughput (=count of RACHs succeeding in access/total count of RACHs allocated to one frame) of the first allocating method is relatively lower than that of another allocating method. In case that the count of the RACHs is set excessively in a prescribed frame, it results in the waste of radio resources. In contrast, if the count of the RACHs is set lower than necessary in a prescribed frame, collisions of the MTs for the access attempt are raised to elongate the overall access time.
And, since the binary exponential back-off algorithm is used as the algorithm for the retransmission of the RR packets, unfair access opportunities are provided to the MTs having the collision occurrence and retransmission delay is considerable.
Moreover, since each of the MTs needs to calculate the window size (=frame count) necessary for the re-attempt of the random access using the binary exponential back-off algorithm, complexity in MT implementation is raised.
In the second allocating method, as the RACH is dynamically allocated using the split algorithm, the MTs having the collision occurrence in attempting the random access succeed in retransmission of the RR packets fast through a prescribed p(>1) frame. Yet, the transmission of RR packet for new random access is not allowed during the time for the retransmission of the RR packet (contention resolution period). Namely, random access of new MTs is not allowed for the n frames corresponding to the period for resolving the collisions occurring in the frame t in FIG. 1 but new random access is allowed in a frame (t+n+1). So, the access delay for new MTs is brought about.
In the third allocating method, since the RACH count of each frame is changed, the resources can be used more efficiently than those of the first allocating method. Yet, since each MT uses the binary exponential back-off algorithm like the first allocating method, the same problem of the second allocating method is brought about.